1. Field of the Invention
The present invention relates to a fuel reforming apparatus which converts a hydrocarbon crude fuel to a hydrogen-rich gaseous fuel through a reforming reaction and feeds a supply of the gaseous fuel to fuel cells. The present invention also pertains to a method of the same and a fuel-cells system with the fuel reforming apparatus incorporated therein.
2. Description of the Prior Art
Fuel cells convert the chemical energy of a fuel not via mechanical energy or thermal energy but directly into electrical energy and thereby realize a high energy efficiency. A well-known structure of the fuel cells includes a pair of electrodes arranged across an electrolyte layer. A supply of hydrogen-containing gaseous fuel is fed to one electrode or a cathode, whereas a supply of oxygen-containing oxidizing gas is fed to the other electrode or an anode. The fuel cells generate an electromotive force through electrochemical reactions proceeding on the electrodes. Equations (1) through (3) given below represent electrochemical reactions proceeding in the fuel cells. Equation (1) shows the reaction proceeding at the cathode, whereas Equation (2) shows the reaction proceeding at the anode. The reaction shown by Equation (3) accordingly proceeds as a whole in the fuel cells.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
xe2x80x83(xc2xd)O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3) 
The fuel cells are classified, for example, by the type of the electrolyte and the driving temperature. An oxidizing gas and a gaseous fuel containing carbon dioxide may be used in polymer electrolyte fuel cells, phosphate fuel cells, and molten carbonate fuel cells, because of their properties of the electrolytes. In these fuel cells, the air is generally used as the oxidizing gas, and the hydrogen-containing gas produced by steam reforming a hydrocarbon crude fuel, such as methanol or natural gas, as the gaseous fuel.
A reformer functioning as the fuel reforming apparatus is incorporated in a fuel-cells system including such fuel cells. The reformer converts the crude fuel into a gaseous fuel through the reforming reactions. By way of example, the following reforming reactions proceed in the reformer to steam reform methanol used as the crude fuel:
CH3OHxe2x86x92CO+2H2xe2x88x9290.0(kJ/mol)xe2x80x83xe2x80x83(4) 
CO+H2Oxe2x86x92CO2+H2+40.5(kJ/mol)xe2x80x83xe2x80x83(5) 
CH3OH+H2Oxe2x86x92CO2+3H2xe2x88x9249.5(kJ/mol)xe2x80x83xe2x80x83(6) 
In the process of steam reforming methanol, the decomposition reaction of methanol expressed by Equation (4) proceeds simultaneously with the conversion reaction of carbon monoxide expressed by Equation (5). The reaction of Equation (6) thus occurs as a whole. Since the reaction for reforming the crude fuel is endothermic, an external heating unit, such as a burner or a heater, is attached to the reformer to supply the heat required for the endothermic reforming reaction.
When the heat required for the reforming reaction is supplied externally to the reformer, a large portion of the supply of heat is not used for the reforming reaction but is wasted. This lowers the energy efficiency of the whole system with the reformer. When the hot combustion gas from the burner supplies the heat required for the reforming reaction, for example, the hot combustion exhaust containing a considerable quantity of energy that has not been used for the reforming reaction is wastefully discharged from the reformer. In another example, when the heater is used as the heating unit, a considerable quantity of energy produced by the heater is used not to promote the reforming reactions but to heat a reaction vessel of the reformer.
In the method of supplying heat from the burner or the heater, when the quantity of the reforming reactions (that is, the quantity processed through the reforming reactions) is significantly varied with a significant change in amount of the crude fuel fed to the reformer, it is difficult to keep the internal temperature of the reformer within a desirable temperature range suitable for the reforming reactions and ensure the sufficient activity of the reforming reactions. When the amount of the crude fuel, such as methanol, fed to the reformer is increased to increase the quantity processed through the reforming reactions, the internal temperature of the reformer is lowered with the progress of the endothermic reforming reaction. The temperature decrease results in deactivating the reforming reaction. It may be considered that an increase in quantity of heat supplied from the burner or the heater prevents the temperature decrease in the reformer. This method, however, can not sufficiently follow the temperature variation due to an abrupt increase in quantity processed through the reforming reactions, since there is a limit in rate of heat transfer in the reformer.
When the amount of the crude fuel fed to the reformer is decreased to decrease the quantity processed through the reforming reactions, on the other hand, a decrease in heat consumed by the reforming reactions raises the internal temperature of the reformer. In case that the temperature increase causes the internal temperature of the reformer to exceed the desired temperature range, undesired reactions other than the reforming reactions expressed by Equations (4) through (6) given above proceed in the reformer and cause the gaseous fuel to be contaminated with undesirable products. The excessive increase in temperature of the reformer deteriorates the reforming catalyst included in the reformer and shortens the life of the reformer. It may be considered that a decrease in quantity of heat supplied from the burner or the heater prevents the temperature increase in the reformer. This method, however, can not sufficiently follow the temperature variation due to an abrupt decrease in quantity processed through the reforming reactions, since the reformer itself has a predetermined heat capacity.
Another known method of supplying the heat required for the reforming reaction feeds a supply of oxygen-containing oxidizing gas as well as a supply of the crude fuel to the reformer and causes the exothermic oxidation reaction to proceed with the endothermic reforming reaction in the reformer, in order to supply the heat required for the reforming reaction by the heat produced by the oxidation reaction (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 4-160003). A reformer 134 shown in FIG. 5 is an example of such known reformers. The reformer 134 has a reforming reaction unit 180 including a catalyst layer 181. The catalyst layer 181 receives a supply of crude fuel gas containing, for example, methanol and a supply of the air ingested from outside via an air supply unit 190. A temperature sensor 186 is disposed in the catalyst layer 181. The driving state of a flow control valve 192 located in the air supply unit 190 is controlled, based on the temperature in the catalyst layer 181 measured by the temperature sensor 186. The control of the driving state regulates the amount of the air fed to the catalyst layer 181.
The amount of the air supplied to the catalyst layer 181 is regulated according to the observed temperature of the catalyst layer 181. Regulation of the supply of the air fed to the catalyst layer 181 in order to keep the internal temperature of the catalyst layer 181 within a desired temperature range accordingly makes the heat consumed by the reforming reaction well balance the heat produced by the oxidation reaction and ensures the sufficiently high activity of the reforming reactions proceeding in the reformer 134. In the reformer 134 of this structure, the heat required for the reforming reactions is supplied inside the reformer. This structure effectively reduces the quantity of heat that is not used for the reforming reactions but is wasted, and thereby ensures the high energy efficiency.
In the proposed structure where the oxidation reaction proceeds with the reforming reaction in the reformer, however, it is rather difficult to keep the temperature of the whole reformer in a uniform state in the desired temperature range under the condition of a significant change in quantity processed by the reforming reactions. When the quantity processed through the reforming reactions abruptly increases in the reformer 134, both the supply of the air fed from the air supply unit 190 and the supply of the crude fuel gas abruptly increase. This causes the non-uniform temperature distribution in the catalyst layer 181.
With an increase in amount of the air supplied to the catalyst layer 181, the oxidation reaction proceeds actively and quickly raises the catalyst temperature in an upstream area of the catalyst layer 181 having the high concentration of the air (oxygen), that is, the area close to the surface of the catalyst layer 181 which receives the supply of the air from the air supply unit 190. A downstream area of the catalyst layer 181, that is, the area close to the surface of the catalyst layer 181 which discharges the resulting reformed gas after the reforming reaction, on the other hand, has the low concentration of oxygen since oxygen has been consumed by the oxidation reaction on the upstream side. In the downstream area, the steam reforming reaction actively proceeds in preference to the oxidation reaction and lowers the catalyst temperature. Even when the supply of the air is regulated according to the results of detection of the temperature sensor 186, the catalyst temperature is high on the upstream side and gradually decreases along the flow of the gas in the catalyst layer 181. On the upstream side of the catalyst layer 181, the high temperature may cause production of undesired components by the undesired reactions and deterioration of the catalyst. On the downstream side of the catalyst layer 181, on the other hand, the low temperature may cause the lowered activity of the reforming reactions.
One object of the present invention is thus to enhance the utilization efficiency of heat in reforming reactions in a method of reforming fuel as well as in a fuel reforming apparatus and a fuel-cells system with the fuel reforming apparatus incorporated therein.
Another object of the present invention is to keep the activity of the reforming reactions at a sufficiently high level even when the quantity processed through the reforming reactions is varied.
At least part of the above and the other related objects is realized by a method of converting a hydrocarbon included in a crude fuel gas to hydrogen through an endothermic reforming reaction. The method includes the steps of:
(a) feeding a supply of the crude fuel gas to a reformer including a plurality of reforming reaction units, in which the reforming reaction proceeds, the supply of the crude fuel gas successively passing through the plurality of reforming reaction units;
(b) detecting a progress of the reforming reaction in each of the plurality of reforming reaction units;
(c) feeding a supply of oxygen to each of the plurality of reforming reaction units to make an exothermic oxidation reaction proceed in each reforming reaction unit, and causing heat produced by the oxidation reaction to be utilized for the reforming reaction; and
(d) regulating an amount of oxygen fed to each reforming reaction unit, based on the progress of the reforming reaction detected in each reforming reaction unit.
The present invention is also directed to a fuel reforming apparatus which converts a hydrocarbon included in a crude fuel gas to hydrogen through an endothermic reforming reaction and discharges a gaseous fuel containing the hydrogen. The fuel reforming apparatus includes: a reformer including a plurality of reforming reaction units, each reforming reaction unit having a reforming catalyst, which accelerates the reforming reaction, and an oxidation catalyst, which accelerates an exothermic oxidation reaction in the presence of oxygen; a crude fuel supply unit which feeds a supply of the crude fuel gas to the reformer, so as to cause the supply of the crude fuel gas to successively pass through the plurality of reforming reaction units; a progress detection unit which detects a progress of the reforming reaction in each of the plurality of reforming reaction units; an oxygen supply unit which feeds a supply of oxygen to each of the plurality of reforming reaction units to make the oxidation reaction proceed; and an oxygen supply regulation unit which regulates an amount of oxygen fed to each reforming reaction unit via the oxygen supply unit, based on the progress of the reforming reaction detected by the progress detection unit.
In the method of reforming fuel and the corresponding fuel reforming apparatus of the present invention, the reforming reaction proceeds with the heat produced by the oxidation reaction that proceeds in the plurality of reforming reaction units. This structure effectively reduces the quantity of energy consumed for the supply of heat required for the reforming reaction. The amount of oxygen fed to each reforming reaction unit is regulated by the progress of the reforming reaction in the reforming reaction unit. The progress of the reforming reaction is accordingly kept in a desired state by making the relationship between the quantity of the reforming reaction proceeding in the fuel reforming apparatus and the supply of oxygen satisfy a predetermined condition.
Regulation of the amount of oxygen supplied to each reforming reaction unit enables the progress of the reforming reaction to be kept in the desired state. Even when the quantity of the reforming reaction proceeding in the reformer is varied, this structure effectively prevents troubles due to the local temperature increase or the local temperature decrease in the fuel reforming apparatus. The local excess of oxygen supply in a specific area of the fuel reforming apparatus increases the temperature in the specific area and causes production of undesired by-products and deterioration of the catalyst. The local insufficiency of oxygen supply in a specific area, on the contrary, decreases the temperature in the specific area and lowers the activity of the reforming reaction. In the method of reforming fuel and the corresponding fuel reforming apparatus of the present invention, the progress of the reforming reaction is kept in the desired state by regulating the supply of oxygen. This structure favorably prevents the above problems even when the quantity of the reforming reaction proceeding in the reformer is varied.
Since the progress of the reforming reaction is kept in the desired state in each reforming reaction unit, even when the quantity processed through the reforming reaction is increased, each reforming reaction unit is utilized efficiently to promote the reforming reaction. The fuel reforming apparatus of the present invention does not require an excess of the reforming catalyst and thereby reduces its size.
In accordance with one preferable application of the method, the step (b) includes the step of:
detecting the progress of the reforming reaction in each of the plurality of reforming reaction units, based on an internal temperature of each reforming reaction unit, and
the step (d) includes the step of:
regulating the amount of oxygen fed to each reforming reaction unit, in order to keep the internal temperature of each reforming reaction unit in a predetermined temperature range.
In accordance with one preferable application of the fuel reforming apparatus, the progress detection unit detects the progress of the reforming reaction in each of the plurality of reforming reaction units, based on an internal temperature of each reforming reaction unit, and the oxygen supply regulation unit regulates the amount of oxygen fed to each reforming reaction unit, in order to keep the internal temperature of each reforming reaction unit in a predetermined temperature range.
In this preferable structure, the amount of oxygen supplied to each reforming reaction unit is regulated to keep the internal temperature of the reforming reaction unit within the predetermined temperature range. This enables the progress of the reforming reaction to be kept in a desired state and ensures the sufficiently high activity of the reforming reaction.
The present invention is further directed to a fuel-cells system, which includes the fuel reforming apparatus of the present invention and a fuel cell that receives a supply of the gaseous fuel produced by the fuel reforming apparatus and generates electricity.
In the fuel-cells system of this structure, the heat produced by the oxidation reaction is utilized for the reforming reaction in the fuel reforming apparatus. This structure reduces the quantity of energy consumed by the fuel reforming apparatus as the heat required for the reforming reaction and thereby enhances the energy efficiency of the fuel-cells system. Since the progress of the reforming reaction is kept in the desired state in the fuel reforming apparatus, the quantity of the reforming reaction proceeding in the fuel reforming apparatus can be varied with a variation in quantity of power generation in the fuel cells. The reforming catalyst included in the fuel reforming apparatus is utilized at a sufficiently high efficiency, so that the size of the fuel reforming apparatus can be reduced to a sufficient level. These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings.